Puzzles and novelty toys that rely on optical illusions and/or other visual conundrums seem to remain popular with both children and adults. One example of such a puzzle which has recently been marketed under the trade mark MAGIC EYE are the pictures which when viewed normally typically do not appear to represent anything clearly discernible, but when viewed with the left and right eyes slightly out of focus a 3-D image becomes visible.
It appears that the popularity of such puzzles and novelty toys arises from an innate curiosity and fascination with mystery in human beings. The play value of such puzzles and toys lies principally in the ability to solve the riddle or to uncover the mystery in such a way that does not immediately render the puzzle or toy useless. Having solved the conundrum on one occasion, a user can return to the puzzle or toy on another occasion and still derive enjoyment in solving the conundrum a second, a third, or fourth time.